Bosonic Quantum Conditional Mutual Information and Bose-Einstein Condensation

This article aims to introduce the bosonic asymmetric quantum conditional mutual information (QCMI), $I_B(A;B\mathrm{\mid }C)$, as a rigorously defined information-theoretic quantity for continuous-variable (bosonic) systems. After defining $I_B(A;B\mathrm{\mid }C)$ through quantum instruments on infinite-dimensional Fock spaces, we prove that Bose-Einstein condensation (BEC) induces a logarithmic divergence $I_B(A;B\mathrm{\mid }C)\ge \alpha {\log }_2(N_0^C+1)$, which further establishes $I_B(A;B\mathrm{\mid }C)$ as a quantum-information order parameter for BEC with power-law scaling near the critical point. We provide the complete five-step proof of this result, connecting measurement back-action, off-diagonal long-range order, and Jensen’s inequality. In the condensed phase, the effective causal propagation velocity is set by the Bogoliubov sound speed $c_s$ in the long-wavelength limit, and vanishes continuously at the superfluid–Mott-insulator transition. Applications to composite bosons, the BEC–BCS crossover, and a quantum-gas-microscope measurement protocol are presented. Finally, we demonstrate how the reduced bosonic QCMI serves as a finite-size scaling probe analogous to the Binder cumulant.